1. Field of the Invention
The present invention relates to a method of preparing a bismuth oxide superconductor, and more particularly, it relates to a method of preparing a bismuth oxide superconductor, which is capable of improving the critical current density and suitable for preparation of a long wire.
2. Description of the Background Art
In recent years, superconductive materials of ceramics, i.e., oxide superconductive materials, are watched as materials which exhibit higher critical temperatures. For example, a bismuth oxide superconductive material, which exhibits a high critical temperature of about 110 K., is expected as a practicable superconductive material.
It is known that a bismuth oxide superconductor has a phase showing a critical temperature of 110 K. and those showing critical temperatures of 80 K. and 10 K.
In relation to such a bismuth oxide superconductor, it is also known that the 110 K. phase has a 2223 composition in a composition of Bi-Sr-Ca-Cu, or (Bi,Pb)-Sr-Ca-Cu with partial substitution of Pb for Bi, while the 80 K. phase has a 2212 composition of the same elements.
In a method of preparing an oxide superconductor, raw material for the oxide superconductor is filled into a metal sheath and subjected to deformation processing and heat treatment, so that the raw material contained in the metal sheath is brought into a superconducting state. This method is advantageously applied to preparation of a long superconducting wire, for example.
A process of preparing such an oxide superconductor includes at least a heat treatment step, and it has been recognized that a sufficient amount of oxygen must be supplied in such a heat treatment step, in order to improve the critical current density.
In order to shape a superconductor into a long wire and apply the same to a cable or a magnet, it is necessary to attain a high critical current density in addition to a high critical temperature. Further, the critical current density of the wire is preferably as uniform as possible along its overall length.
When a long superconducting wire is to be prepared as described above, the superconductor is covered with a metal sheath, in a structure basically cut off from the external atmosphere. Therefore, the wire shows such a tendency that high critical current densities are attained at its end portions, which are relatively easily influenced by the external atmosphere, while the critical current density is reduced at its central portion.
Further, the wire may be swelled by a gas which is generated from the raw material powder during heat treatment, to cause reduction of the critical current density. Although it has been recognized possible to prevent damage resulting from such swelling of the wire by performing the heat treatment under decompression, the critical current density is further remarkably reduced in this case due to oxygen deficiency caused by decompression.